Students will soon return to Manse Elementary School in Pahrump, Nev., so Judith Lister has been getting her kindergarten classroom ready, filling the bookshelves and arranging floor pillows for read-aloud time. She’s about to tackle the bulletin boards.

“I’m looking forward to it, but you have to be physically active,” Ms. Lister said. “Getting up off the floor is sometimes a trick.” She has found that a rolling adjustable stool helps.

At 71, Ms. Lister isn’t ready to retire. It’s an increasingly popular decision: Older people are staying in the work force longer.